1. Field of the Invention
The present invention relates to a system for purifying an exhaust gas for a diesel engine, particularly to a system for trapping and incinerating particulates in the exhaust gas.
2. Description of the Related Art
Known in the prior art is a system for purifying particulates in an exhaust gas for a diesel engine provided with a filter arranged in an exhaust line of the engine. The particulates in the exhaust gas are caught and held by the filter.
Two types of filter are known. The first is a honey-comb type made of perforated ceramic material having a plurality of juxtaposed passageways having ends alternately plugged, wherein the exhaust gas is introduced into the passageways of the closed downstream end, passed through the wall thereof, and then discharged through the passageways of the closed upstream ends. The second is a foamed filter made of foamed ceramic material having a three-dimensional mesh construction. Irrespective of the type thereof, the filter is arranged in the casing connected to the exhaust pipe of the diesel engine, and a heater is arranged in the casing at a position near the inlet of the filter.
When a predetermined amount of particulates has accumulated in the filter, the heater is operated to ignite and incinerate the accumulated particulates, to refresh the filter. To detect the timing at which the refresh operation of the filter should be commenced, a system has been proposed in Japanese Unexamined Patent Publication (Kokai) No. 59-126018, wherein a value of a pressure difference across the filter is detected. The determination of the timing at which the refresh operation should be commenced is effected by determining whether the pressure difference is larger than a predetermined value. When the pressure difference becomes larger than the predetermined difference value, the heater is operated to start the refresh operation of the filter.
The detection of the pressure difference across the filter is suitable for the above mentioned honey-comb type filter for determining a correct timing for commencing the refresh operation of the filter. In this type of the filter, the exhaust gas is introduced to an upstream end of each passageway having a closed downstream end, and is discharged via a downstream end of each passageway having a closed upstream end. This type of filter also has a plurality of fine voids in the walls between the passageways, through which voids the exhaust gas is passed. When the exhaust gas is passed through the voids, the particulates are deposited on the surface of the walls and thus do not pass therethrough. When the amount of the deposited particulates is increased, the pressure difference across the filter is also naturally increased. Therefore, the value of the pressure difference across the filter corresponds precisely to the amount of particulates trapped by the filter.
When the above mentioned foamed type filter is employed, the pressure difference does not always precisely indicate the amount of trapped particulates. This type of filter has the three-dimensional mesh construction, and has voids having a diameter which is larger than that of the voids in the honey-comb type filter. From the dimensional viewpoint, the particulates can easily pass through the filter, but in this type of filter, when the exhaust gas is passed through the three-dimensional mesh construction, the particulates come into contact with the inner surface of the mesh construction, and are "adsorbed" thereon. This trapping of the particulates by adsorption on the surface of the three-dimensional mesh structure in the foamed type filter has a lower trapping efficiency and there is little or no pressure drop across the filter. Accordingly, in the foam type filter, the pressure difference across the filter does not always directly correspond to the amount of trapped particulates, and as a result, a detection of the timing of the refresh operation of the filter is sometimes imprecise and unsatisfactory.
In a known prior art system provided with a smoke density sensor, the timing for a refresh operation of a filter is detected by judging whether an accumulated value of the smoke density has reached a predetermined value. (See Japanese Unexamined Patent Publication (Kokai) No. 59-150917.) The accumulated value of the smoke density in this prior art can not detect a correct timing for the refresh operation of the filter, since the trapping ratio itself changes when the amount of particulates held in the filter is varied.
In this prior art, the value of the smoke density is accumulated when the filter is refreshed by itself. Thus, the refresh operation is prematurely commenced while the filter can still trap a substantial amount of particulates.